A conventional gas spring for use in various machines is provided with a cylinder main body, a rod member slidably fitted to the cylinder main body, a gas filled chamber inside of the cylinder main body and filled with compressed gas (i.e. nitrogen gas). Compressed gas (pressure is about 7 to 10 MPa), is filled in the gas filled chamber. The rod member is driven to advance by the compressed gas. Here, the gas spring is provided with various mechanisms for restricting the advancing operation of the rod member in such a manner that the rod member cannot advance in excess of a predetermined stroke.
For example, the applicant of the present invention has proposed a gas spring as shown in FIG. 10. The gas spring 100 comprises a cylinder main body 101; a rod member 102 with a cylindrical piston penetrating the cylinder main body 101; a gas filled chamber 103, which is formed between the cylinder main body 101 and the rod member 102 and urges the rod member 102 upward with pressure of compressed gas filled inside; a plurality of pin members 104 disposed symmetrically with each other around an axial center of the rod member 102; and slots 105 which receive pin members 104, so as to restrict the moving range of the rod member 102 with respect to the cylinder main body 101. When the rod member 102 advances upward by a predetermined stroke, each pin member 104 fits to the upper end of the slot 105, thereby restricting the movement of the rod member 102.
In the above-described gas spring, after filling of the compressed gas into the gas filled chamber 103, the rod member 102 is pressed down with respect to the cylinder main body 101, to thus turn the gas filled chamber 103 into a sealed state. Thereafter, it is necessary to securely screw the pin members 104 to a screw hole formed at the rod member 102 in order to restrict the movement of the rod member 102. As a result, the man-hour of a gas filling work is increased. Moreover, there is a possibility that the structure of the gas spring becomes complicated.
An object of the present invention is to constitute a gas spring such that a rod member can be restricted at an upper limit position at the same time when compressed gas is filled into a gas filled chamber, and to simplify a mechanism for restricting the advancing motion of the rod member.